Heretofore, it has been known, as evidenced by the development of the field of cermets that, generally, a mixture of ceramic and metallic components into a single product may, and often does, result in such product having physical properties not found solely in either one of the components. Generally, such products combine strength of the metal with the heat, wear and oxidation resistance of the ceramic material.
One of the most difficult problems in the field of cermets (sometimes referred to as metal-reinforced or metal-filled ceramics) is to reduce or eliminate porosity within the resulting composite material.
Generally, in the past, cermets were made by sintering a compacted mixture of metal and ceramic powders. However, during the sintering process an oxide film forms on the metal and inhibits its complete infiltration into the resulting ceramic matrix. Such a method, more often than not, produces metal-reinforced ceramics with 10% to 15% void space volumes. The void spaces decrease the effective cross-sectional area of the cermet and act as stress concentrators. Consequently, such prior art cermets, having such undesirable voids, are often limited in their use to applications not requiring high material strength and impact resistance.
The prior art has attempted to overcome such problems by proposing various methods by which, hopefully, the void spaces would be reduced or eliminated. One such method proposed by the prior art includes taking ceramic powder and first pressing it to form a porous compact of a desired configuration. The compact and a quantity of filler metal are placed within a vacuum chamber which, in turn, is heated to a temperature above the melting point of the metal (often to one and a half times the metal melting temperature) and is evacuated to a pressure below 10.sup.-.sup.6 torr. The vacuum encourages the dissociation of the oxide film from the metal surface allowing the molten metal to flow more freely into the void spaces of the ceramic compact.
Some cermets or metal-filled ceramics formed by the above vacuum impregnation process have exhibited a porosity, in terms of void spaces, of only three percent. However, the prior art has also indicated that to achieve even such results the attainment of a vacuum of less than 10.sup.-.sup.6 torr. is essential. With a "torr" being defined as 1/760 of an atmosphere, it can be seen that the above vacuum impregnation process requires ultra-high vacuum so that the pressure within the vacuum chamber would be in the order of or less than 0.0000000193 psia. Obviously, the attainment of such a high vacuum requires sophisticated equipment which, when combined with the temperature requirements of the process preclude the ready adoption of the vacuum impregnation process to rapid production requirements, especially when it is remembered that the resulting cermet still contains a significant porosity in terms of void spaces.
The prior art has also reasoned that when cermets are formed as by the application of heat and pressure to an admixture of powdered ceramic material and powdered metal, void spaces created therein result from the frictional resistance afforded by the individual particles of powdered metal and powdered ceramic material during the application of pressure thereto. That is, the frictional resistance (or interference caused by the particular geometric configuration of such particles) prevents the powder particles from moving sufficiently freely with respect to each other as to minimize or eliminate significant void spaces therebetween. The prior art has suggested that this problem of void spaces, if caused in accordance with the above postulated theory, could be overcome as by the addition of suitable adjuvants to the admixture of powdered metal and ceramic material. However, this method has not been accepted, especially beyond laboratory requirements such as in commercial production, because of additional problems. That is, the inclusion of pressing adjuvants requires adjuvants of essentially highest purity and is further limited to that group of additives which are not reduceable oxides since any substantial quantity of impurity causes the mechanical equivalent of a void space at the location of such impurity within the resulting cermet while such oxides, because of the temperatures required of the process, chemically react with the primary metal or metal alloy of the resulting cermet forming inclusions, which effectively reduce the strength of the cermet.
Accordingly, the invention as herein disclosed and described is primarily directed to the solution of the above as well as other related problems.